1. Field of the Invention
The present invention relates to a very high frequency tuner, and more specifically relates to an improvement in a tuned coupling circuit between a high frequency amplifier and a frequency converter in a very high frequency tuner, particularly for use in television receivers.
2. Description of the Prior Art
A typical prior art very high frequency tuner comprises a high frequency amplifier and a frequency converter, with a tuned coupling means interposed therebetween. One type of such a tuned coupling means comprises a double tuned coupling circuit i.e. a pair of inductively coupled tuned circuits. The other type of such a tuned coupling means comprises a single tuned coupling circuit. In comparison of these, a single tuned circuit is more advantageous from the standpoint of economy and fabrication of the same.
FIG. 1 shows a schematic diagram of only a major portion of a typical conventional turret type tuner for use in a very high frequency or VHF television receiver. Referring to FIG. 1, the tuner is shown comprising a transistor TR1 implementing a high frequency amplifier and a transistor TR2 implementing a frequency converter, interconnected by a single tuned coupling circuit comprising an inductance coil and capacitors connected thereto. The said single tuned coupling circuit comprises a selected one of tuning inductance coils L1, L2, L3, . . . , which is connected at one terminal a to a common bus 1' through a capacitor C3 and is shunted at the other terminal b by a series connection of capacitors C1 and C2. More specifically, the inductance coils L1, L2, L3 . . . are provided on a turret plate such that each is individually associated with each channel, whereby rotation of the tuner shaft makes selective connection of each coil to the terminals a and b. The transistor TR1 comprises a typical bipolar type transistor of NPN type having collector, base and emitter electrodes and is connected in a common emitter fashion. One terminal b for the coil is connected to the collector electrode of the transistor TR1, while the other terminal a of the coil is connected through the capacitor C3 to the common bus 1'. The base electrode serving as an input electrode is connected to the input terminal 1. It is well known to those skilled in the art that in a high frequency amplifier employing a transistor connected in a common emitter fashion a negative feedback loop is formed through a stray capacitance between the base and collector electrodes and hence a neutralizing signal obtainable from the output is applied to the input electrode for the purpose of neutralizing the said feedback operation through the stray capacitance between the base and collector electrodes. In the FIG. 1 diagram, the capacitor C3 has been inserted for the purpose of obtaining such a neutralizing signal. The neutralizing signal obtained at the junction a of the capacitor C3 and the inductance coil is applied through a neutralizing capacitor C4 to the base electrode of the transistor TR1. The coil to be selected at the terminals a and b and the capacitor C3 connected thereto are shunted by a series connection of the capacitors C1 and C2 provided for the purpose of capacitance division for applying the output to the succeeding stage. It would be appreciated that the coil selected at the terminals a and b and the capacitors C1, C2 and C3 to be connected in series with the coil constitute a tuned or resonance circuit. The transistor TR2 also comprises a typical bipolar NPN type transistor having collector, base and emitter electrodes connected in a common emitter fashion. The output signal obtainable from the preceding stage at the capacitance divided terminal is applied to the base electrode of the transistor TR2. The base electrode of the transistor TR2 is also connected from a local oscillator, not shown, through a capacitor C5 and is connected to a succeeding intermediate frequency amplifying circuit through a capacitor C6. Returning again to the transistor TR1, the high frequency amplifying transistor TR1 is typically connected to form an amplifier of an automatic gain control type. To that end, the base electrode of the transistor TR1 is connected to receive an automatic gain control signal from a terminal AGC. In this connection it should be pointed out that the transistor TR1 comprises a stray capacitance between the collector and emitter electrodes which is variable as a function of the magnitude of the automatic gain control signal to be applied to the transistor TR1, as to be more fully discussed subsequently.
FIG. 2 shows a simplified schematic diagram of the FIG. 1 tuner where a specified channel is selected, wherein the neutralizing circuit has been omitted for simplicity of consideration of the circuit. Since the same portions have been denoted by the same reference characters, the circuit connection is self-explanatory. Now let it be assumed that a signal of the frequency much higher than the frequency of a desired receiving channel is applied to the input terminals 1 and 1'. Then the impedance of the inductance coil L1 with respect to such a high frequency interference signal becomes accordingly such larger. Therefore, it follows that the inductance coil L1 becomes negligible with respect to such a high frequency interference signal. FIG. 3 shows an equivalent circuit of the FIG. 2 diagram with respect to such a high frequency interference signal wherein the inductance coil L1 has been omitted. Referring to FIG. 3, it would be appreciated that such a high frequency interference signal obtainable from the high frequency amplifying transistor TR1 is capacitance divided at the rate of C1/C1+C2 and is transferred to the succeeding frequency converting transistor TR2. Thus a conventional VHF tuner employing a single tuned coupling circuit suffers from the shortcomings that such a high frequency interference signal of the frequency much higher than the receiving frequency is transferred through the coupling circuit from the high frequency amplifying transistor TR1 to the frequency converting transistor TR2 but nevertheless it is difficult to block such a high frequency interference signal.
Transmission of such an interference signal to the frequency converting stage will now be more specifically considered. Assuming that the central frequency f1 of a given low channel in the VHF television channels is received, the local oscillating frequency f0 of the tuner is selected with respect to the central frequency f1 of the said broadcasting channel to the value that meets the relation of f0-f1=56.5 MHz which is commensurate with the intermediate frequency of the receiver in case of the Japanese standard television system. Such intermediate frequency signal thus obtained based upon the said desired broadcasting signal is applied to the succeeding video intermediate frequency circuit in the television receiver. As well known to those skilled in the art, such frequency conversion is often referred to as the upper side superheterodyne. Assuming that in such a situation a broadcasting signal in a certain high band of the frequency f2 that meets the relation of f2-f0=56.5 MHz is received by the high frequency amplifying transistor TR1 and the output therefrom is applied to the frequency converting transistor TR2, it follows that the intermediate frequency signal obtained based upon the said high band channel signal is also applied to the succeeding video intermediate frequency circuit. Such intermediate frequency signal obtained based upon the undesired high band channel signal could cause an interference on the image appearing on the screen of the television receiver. Taking an example, the signal of the channels Nos. 10 through 12 could cause such an interference when the signal of the channels Nos. 1 through 3 is received. Thus, it would be appreciated that it is necessary that consideration is given to prevent or block such a high band channel signal of the frequency f2 from being transferred to the frequency converting transistor TR2. The same also applies to a ultra high frequency or UHF broadcasting signal the frequency of which is different by 56.5 MHz from the harmonic frequency nf0 of the local oscillating frequency f0. Hence, it is also required that such a UHF signal is also prevented from being transferred to the frequency converting stage for the same reason.
Another serious problem countered with a VHF tuner employing a single tuned coupling circuit interposed between a high frequency amplifying transistor and a succeeding frequency converting transistor is an adverse effect of the output stray capacitance of the high frequency amplifying transistor upon the tuned circuit and thus to the tuning frequency of the single tuned coupling circuit. Such an adverse effect caused by an output stray capacitance of the high frequency amlifying transistor in the tuner will now be more specifically considered. As well known to those skilled in the art and as briefly discussed in the foregoing, a typical conventional VHF tuner comprising a high frequency amplifying transistor and a frequency converter with a coupling circuit interposed therebetween is supplied with an automatic gain control signal, so that the gain in the tuner is automatically controlled as a function of the automatic gain control signal. Again referring to FIG. 1, such an automatic gain control signal is typically applied through the terminal AGC to the input electrode, i.e. the base electrode of the high frequency amplifying transistor TR1. On the other hand, the transistor TR1 comprises a stray capacitance between the output electrode, i.e. the collector electrode and the common electrode, i.e. the emitter electrode. It is well known to those skilled in the art that such a stray capacitance of the transistor TR1 is varied as a function of the magnitude of the automatic gain control signal to be applied to the input electrode of the transistor TR1. Such a stray capacitance serves to shunt the inductance coil L1 together with the series connection of the capacitors C1 and C2. Thus, the tuning frequency of the single tuned coupling circuit is determined by the values of the inductance of the coil L1 and the resultant capacitance of the capacitors C1, C2 and C3 and the said stray capacitance. In considering the tuning frequency of the tuned circuit, since the capacitance of the capacitor C3 is relatively large, the resultant capacitance of the capacitors C1 and C2 and the said tray capacitance is dominant. Since the tuned coupling circuit shown in FIGS. 1 and 2 comprises a series connection of the capacitors C1 and C2 shunting the inductance coil L1, the resultant capacitance of the series connected capacitors C1 and C2 is calculated as C1.multidot.C2/C1+C2 and becomes relatively small as compared with the capacitance value of each capacitor. Variation of the said stray capacitance causes an adverse affect upon the tuning frequency of the tuned coupling circuit at the rate of the stray capacitance to the said resultant capacitance of a relatively small value. Thus in order to reduce any adverse affect by the said stray capacitance at the output end of the high frequency amplifying transistor, it is desired to select a larger resultant capacitance value of the said series connection of the capacitors C1 and C2, insofar as it is possible. Nevertheless, in order to adopt a larger resultant capacitance value of these capacitors C1 and C2, it is necessary to select a smaller inductance value of the coil L1 to obtain a predetermined tuning frequency. As readily understood, there is a limit in selecting a smaller inductance value of the coil L1 from the standpoint of fabrication. Thus, in designing a VHF tuner employing a single tuned coupling circuit, a dilemma is encountered that a larger resultant capacitance of the capacitors C1 and C2 is desired but nevertheless there is a limit from the standpoint of fabrication of the inductance coil L1.
A television tuner of interest is disclosed in the U.S. Pat. No. 3,305,784, issued Feb. 21, 1967 to Chalmers et al. The referenced patent depicts a UHF television tuner comprising a double tuned coupling circuit, i.e. a pair of inductively coupled tuned circuits. Each of these tuned circuits is shown comprising an inductance coil arranged for inductive mutual coupling which is connected to a .pi. type circuit comprising another inductance coil interposed in a series fashion both ends of which are connected through capacitors to the ground in a shunt fashion. Thus it would be appreciated that the .pi. type circuit is employed in each of tuned circuits as a tuning circuit component cooperatively coupled to the said first mentioned inductive coupling inductance coil. Hence the reference patent fails to contemplate simplification of the tuned coupling circuit in that a double tuned coupling circuit has been employed. Moreover, the reference patent fails to teach or suggest an improvement in an adverse affect of the stray capacitance at the output of the high frequency amplifying circuit upon the tuning frequency of the tuned circuit caused by variation of the magnitude of the automatic gain control signal to be applied to the high frequency amplifier.